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Continuous Measurements of Nitrous Oxide Isotopomers During Incubation Experiments

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Continuous Measurements of Nitrous Oxide Isotopomers During Incubation Experiments Biogeosciences Discuss., doi:10.5194/bg-2017-171, 2017 Manuscript under review for journal Biogeosciences Discussion started: 16 May 2017 c Author(s) 2017. CC-BY 3.0 License. Continuous measurements of nitrous oxide isotopomers during incubation experiments Malte Winther1, David Balslev-Harder1,2, Søren Christensen3, Anders Priemé4,5, Bo Elberling5, Eric Crosson6, and Thomas Blunier1 1Centre for Ice and Climate, Niels Bohr Institute, University of Copenhagen, Denmark 2DFM - Danish National Metrology Institute, Kgs. Lyngby, Denmark 3Section for Terrestrial Ecology, Department of Biology, University of Copenhagen, Denmark 4Section for Microbiology, Department of Biology, University of Copenhagen, Denmark 5Center for Permafrost, Department of Geosciences and Natural Resource Management, University of Copenhagen, Denmark 6Picarro Inc, Santa Clara, CA 95054 USA Correspondence to: Malte Winther ([email protected]) Abstract. Nitrous oxide (N2O) is an important and strong greenhouse gas in the atmosphere. It is produced by microbes during nitrification and denitrification in terrestrial and aquatic ecosystems. The main sinks for N2O are turnover by denitrification and photolysis and photo-oxidation in the stratosphere. In the linear N=N=O molecule 15N substitution is possible in two distinct positions, central and terminal. The respective molecules, 14N15N16O and 15N14N16O, are called isotopomers. It has 5 been demonstrated that N2O produced by nitrifying or denitrifying microbes exhibits a different relative abundance of the isotopomers. Therefore, measurements of the site preference (difference in the abundance of the two isotopomers) in N2O can be used to determine the source of N2O i.e. nitrification or denitrification. Recent instrument development allows for 15 continuous position dependent δ N measurements at N2O concentrations relevant for studies of atmospheric chemistry. We present results from continuous incubation experiments with denitrifying bacteria, Pseudomonas fluorescens (producing and 10 reducing N2O) and Pseudomonas chlororaphis (only producing N2O). The continuous analysis of N2O isotopomers reveal the transient pattern (KNO to N O and N , respectively). We find bulk isotopic fractionation of -5.01 ‰ 1.20 for P. 3 2 2 ± chlororaphis, in line with previous results for production from denitrification. For P. fluorescens, the bulk isotopic fractionation during production of N O is -52.21 ‰ 9.28 and 8.77 ‰ 4.49 during N O reduction. 2 ± ± 2 The SP isotopic fractionation for P. chlororaphis is -3.42 ‰ 1.69. For P. fluorescens, the calculations result in SP isotopic ± 15 fractionation values of 5.73 ‰ 5.26 during production of N O and 2.41 ‰ 3.04 during reduction of N O. We interpret the ± 2 ± 2 slightly increased isotopic fractionation during reduction to diffusive isotopic fractionation and a difference in active enzymes during production of N2O. In summary, we implemented continuous measurements of N2O isotopomers during incubation of denitrifying bacteria and believe that similar experiments will lead to a better understanding of denitrifying bacteria and N2O turnover in soils and sediments and ultimately hands-on knowledge on the biotic mechanisms behind greenhouse gas exchange 20 of the globe. Keywords 1 Biogeosciences Discuss., doi:10.5194/bg-2017-171, 2017 Manuscript under review for journal Biogeosciences Discussion started: 16 May 2017 c Author(s) 2017. CC-BY 3.0 License. Nitrous oxide, isotopomers, site preference, greenhouse gas, denitrification, Pseudomonas fluorescens, Pseudomonas chloro- raphis 1 Introduction The atmospheric concentration of nitrous oxide (N2O) has increased from approximately 271 ppb before the industrialization 5 to 324 ppb in 2011 (Ciais et al., 2013). This increase has resulted in (1) N2O being the third most important greenhouse gas, that is N2O has the third highest contribution to the radiative forcing of the naturally occurring greenhouse gases (Hartmann et al., 2013), and (2) an increased production of nitrogen oxides (NOx) in the stratosphere and thereby an increased ozone- depletion (Forster et al., 2007; Kim and Craig, 1993). Ice core records show that N2O concentrations positively correlate with northern hemispheric temperature variations, e.g. 10 during the last glacial-interglacial termination as well as over the rapid climate variations occurring during the glacial period, known as Dansgaard-Oeschger events (D-O events) (Schilt et al., 2010). However, occasionally (e.g. D-O event 15 and 17) the N2O concentration increases long before the onset of the dramatic temperature change (Schilt et al., 2010), providing a potential early warning for rapid climate change. Isotopomers of N2O provide information on the sources (Pérez et al., 2000, 2001; Park et al., 2011) i.e. whether N2O originates predominately from nitrification or denitrification processes. As 15 the conditions/ circumstances leading to emissions from the two processes differ both for the marine and terrestrial sources measuring isotopomers potentially improves our understanding of the climate conditions leading to the release of N2O over rapid climatic changes. 15 The N2O molecule has an asymmetric linear structure (N=N=O) where the position of the N can be discriminated. The 15 α 15 β 14 15 16 15 14 16 position in the N2O molecule are named N and N or short α and β for N N O and N N O, respectively 20 (Yoshida and Toyoda, 2000). The two isotopomers can be distinguished by isotope ratio mass spectrometry, if measurements of the NO fragment are included. A distinction is furthermore possible using mid-infrared spectroscopy because the rotational and vibrational conditions are different for the two isotopomers providing spectral regions where absorptions of the two iso- topomers do not overlap (Waechter et al., 2008; Mohn et al., 2010, 2012; Köster et al., 2013b; Heil et al., 2014). For isotopomer measurements at low N2O concentration (low ppm range), a joint instrument development was executed, applying cavity ring 25 down spectroscopy (CRDS) to enable continuous measurements of the isotopomer abundances and yielding values for 15Nα and 15Nβ. The isotopic composition of a sample is reported as delta values which represents the deviation of the elemental isotope ratio 15 α Rsample in the sample from a standard Rstd (Eq. 1). Delta values can be calculated for bulk N2O as well as for δ N and δ15Nβ. All results are reported relative to the isotopic composition of atmospheric nitrogen. 15 15 RSample N 30 δ N = 1 where R = 14 (1) RStd − [ N] 2 Biogeosciences Discuss., doi:10.5194/bg-2017-171, 2017 Manuscript under review for journal Biogeosciences Discussion started: 16 May 2017 c Author(s) 2017. CC-BY 3.0 License. 15 α 15 β The N2O bulk isotopic composition calculates as the average of δ N and δ N (Eq. 2) while the site preference (SP) is by definition their difference (Eq. 2) (Brenninkmeijer and Röckmann, 1999; Toyoda et al., 2002). δ15N α + δ15N β SP = δ15N α δ15N β , δ15N bulk = (2) − 2 There are multiple natural and anthropogenic sources of N2O. The primary anthropogenic sources of N2O are organic and 5 inorganic N fertilizers used for agriculture. The natural sources are primarily nitrification and denitrification in terrestrial and aquatic ecosystems (Mosier et al., 1998; Olivier et al., 1998). Denitrification is a stepwise biological reduction process in which denitrifying bacteria ultimately produce nitrogen (N2). Under anaerobic conditions the denitrifying bacteria use nitrate (NO3−) instead of oxygen as an electron acceptor in the respi- ration of organic matter. Through multiple anaerobic reactions N2 is produced as the end product of the complete denitrifying 10 process (reaction R1) (Firestone and Davidson, 1989). NO− NO− NO N O N (R1) 3 → 2 → → 2 → 2 Each of these anaerobic reactions is carried out by a genuine enzyme, i.e., the production of N2O is caused by the reaction between nitric oxide (NO) and the enzyme nitric oxide reductase (NOR). The NOR enzyme works as a catalyst in the reduction of NO as shown in reaction R2 (Wrage et al., 2001; Tosha and Shiro, 2013). + 15 2NO + 2e− + 2H N O + H O (R2) → 2 2 The cleavage of the covalent N=O bond of N2O leading to N2 and H2O is the result of N2O reduction during bacterial denitrification (R3). According to kinetic isotope theory, the cleavage of N2O is expected to have an increased fractionation effect on 15Nα, due to a stronger 15N O bond compared to the 14N O (Popp et al., 2002), diffusion into the cell (Tilsner − − et al., 2003), and enzymatic reduction (Wrage et al., 2004). N2O reduction during bacterial denitrification is therefore expected 20 to lead to an increase in SP. + N O + 2e− + 2H N + H O (R3) 2 → 2 2 Reactions with different enzymes typically result in specific isotopic fractionation. The isotopic composition of the interme- diately produced N2O during denitrification is a consequence of multiple reaction steps. Two species of denitrifying bacteria with slightly different enzymes potentially leads to different fractionation. In this study, we compared the fractionation of 25 two contrasting denitrifying bacteria; Pseudomonas fluorescens producing and reducing N2O, and Pseudomonas chlororaphis producing but not reducing N2O. 2 Method Our objective was to perform continuous position dependent δ15N measurements of two different bacterial cultures during incubation experiments. Using two denitrifying bacterial cultures we determined the isotopic fractionation and SP during 30 production and reduction of N2O, respectively. 3 Biogeosciences Discuss., doi:10.5194/bg-2017-171, 2017 Manuscript under review for journal Biogeosciences Discussion started: 16 May 2017 c Author(s) 2017. CC-BY 3.0 License. 2.1 Instrumentation Bacterial production of N2O was continuously measured by mid-infrared cavity ringdown spectroscopy using a prototype of the Picarro G5101-i analyzer (in the following named G5101i-CIC) (Picarro, Santa Clara, California, USA). The measurements are non-destructive and are therefore suitable for incubation experiments. The CRDS instrument measures the 14N14N16O, 15 α 15 β 1 1 5 N and N absorption features of N2O in the wavelength region between 2187.4 cm− and 2188 cm− (Balslev-Clausen, 2011).
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